The present invention relates to an apparatus for measuring stress, and more particularly to a fiber-optic add-drop wavelength filter which drops specific channels or adds specific channels in a wavelength division optical communication system while having low insertion loss.
In general, a passive device is required which may drop or add channels at any points in line in order to realize a wavelength division optical communication system. In particular, an element is required which can select narrow wavelength range less than 1 nm while having low insertion loss in order to increase the number of channels. In an optical fiber, fiber-optic Bragg gratings have a characteristic of a good wavelength selectivity and low loss like this.
The simplest fiber-optic add-drop wavelength filter is made by combining fiber-opticr Bragg gratings and a fiber-optic directional coupler. In the fiber-optic add-drop wavelength filter, output optical signals are obtained by inputting optical signals to a port of the fiber-optic directional coupler, reflecting output optical signals therefrom at the fiber-optic Bragg gratings, and then causing the signals to pass through the fiber-optic directional coupler again. This type of add-drop wavelength filter may be very simply manufactured but has a disadvantage of always giving an insertion loss of at least 6 dB. To overcome this, for instance, a non-reciprocal optical circulator is connected to optical Bragg gratings so that input optical signals, after being reflected on the Bragg gratings via the optical circulator, are output through a remaining port through which the optical signals did not proceed in an optical circulator. Although advantageously this is very stable and has small amount of reflection loss and interference between channels, this has disadvantages that there exists a certain insertion loss, an expansion to an integrated optical device is impossible, and the cost is high because this is not an all-fiber device. Therefore, an add-drop wavelength filter is required which is an all-fiber structure while having low loss, and several types of all-fiber add-drop wavelength filters have been proposed.
FIGS. 1A to 1C show functions of structural parts of general add-drop wavelength filters. FIG. 1A shows the function of a wavelength dropping device 10, FIG. 1B shows the function of a wavelength adding device 10xe2x80x2, and FIG. 1C shows the function of an add-drop wavelength filter 10xe2x80x3. Referring to FIG. 1A, when optical signals 60 comprising of multi-wavelengths including a wavelength xcexi are introduced to an input port 20 of the wavelength dropping device, signals of the specific wavelength xcexi propagate to a drop port 40 and the remaining signals excluding the wavelength xcex1 propagate to an output port 30. Referring to FIG. 1B, signals 90 excluding the specific wavelength xcexi are introduced into an input port 20xe2x80x2, added to the signals 110 of xcexi introduced to an add port 50xe2x80x2, and then the added signals 100 propagate to an output port 30xe2x80x2. Referring to FIG. 1C, when optical signals 120 are introduced into an add-drop wavelength filter 10xe2x80x3 through an input port 20xe2x80x3, signals 140 of xcexi in the optical signals are separated to propagate to a drop port 40xe2x80x3, and the remaining signals with the signals of xcexi being separated are joined with new signals 150 of xcexi to propagate to an output port 30xe2x80x3 as new signals 130.
Several configurations of conventional add-drop wavelength filters performing functions like these are illustrated as follows:
FIG. 2 shows a configuration of an add-drop wavelength filter in which Bragg gratings 220 and 222 are written respectively within both arms of a Mach-Zehnder interferometer 205 comprising two optical couplers 210 and 212. When input optical signals are introduced into an input port I, the signals which are separated after the reflection by the Bragg gratings are output through a drop port D, the remaining output signals propagate to an output port D, and optical signals to be added are input into an add port A. An add-drop wavelength filter of this type has a low insertion loss and an excellent wavelength selectivity however requires accurate finish processes about the length or the refractive index of the optical fiber in manufacturing. Furthermore, even after completion, the filter is sensitive to external temperature variation and thus has a poor stability in operation.
FIG. 3 shows a configuration of an add-drop wavelength filter of the type which combines a fiber-optic polarization splitter 230, polarization controllers 240 and 242, and fiber-optic Bragg gratings 250 and 252. FIG. 4 to FIG. 6 show configurations of fiber-optic add-drop wavelength filters of the type in which a fiber-optic Bragg grating 270 is written within a fiber-optic directional coupler 260. Herein, same components are represented as same reference numerals. As described above, although the way using a polarization divider or using a directional coupler, within which a Bragg grating is written, is relatively more stable than an interferometer type, each components should be controlled very accurately in order to attain desired wavelength characteristics and it is difficult to obtain good wavelength characteristics.
Besides, as illustrated in FIG. 7, an add-drop wavelength filter was also presented, in which a titled Bragg grating 310 is written within a fused-type fiber-optic directional coupler 300 which is made with two different optical fibers 280 and 290. In turn, as illustrated in FIG. 8, an add-drop wavelength filter was also presented which was made by using a dual core optical fiber 320, directional couplers 330 and 332, and a Bragg grating 350 which is written within only one core 340 of the dual core optical fiber. But in these cases, they are disadvantageously difficult to manufacture because the used fiber-optic devices can not be made or gained easily.
Furthermore, recently, an add-drop wavelength filter using two different dual mode optical waveguides 360 and 370, mode discriminating directional couplers 380 and 382 and titled Bragg gratings 390, 392 and 394 was proposed by Strasser et al, as shown in FIG. 9. This add-drop wavelength filter has a low loss and is stable in general, but includes several things difficult to be realized in practice. In order to make a mode discriminating directional coupler which is used in the proposed add-drop wavelength filter, two different dual mode optical fibers or dual mode optical waveguides are required in which effective refractive indexes of LP11 mode are same each other with an accuracy of at most 0.0001 and effective refractive indexes of LP01 modes are different from each other, which are difficult to manufacture. Furthermore, two Bragg gratings having same reflective spectra and same mode conversion features are required to be written within two different waveguides respectively, which is also difficult to be realized because a very high accuracy is demanded.
Therefore, in the prior art, it is hard to recognize that the optimum fiber-optic add-drop wavelength filter for a wavelength division optical communication system exists which is easy to manufacture and has low insertion loss and good stability, and therefore there exist difficulties to select a suitable fiber-optic add-drop wavelength filter according to situations.
Accordingly, it is an object of the present invention to provide an all-fiber add-drop wavelength filter which may be easily manufactured while having excellent wavelength selectivity, good stability, and low insertion loss.
The fiber-optic add-drop wavelength filter of the present invention to achieve the above mentioned object basically serves to separate optical signals having at least one wavelength xcexi (1xe2x89xa6ixe2x89xa6n) from input optical signals consisting of multiple wavelengths of xcexi, . . . , and xcexn (n is a positive integer more than 1) or serves to add optical signals having at least one wavelength xcexj to the input optical signals. In the optical add-drop wavelength filter of the present invention, input of the multiple wavelength optical signals and output of the optical signals which experienced wavelength add/drop are performed in a mutually exchanging manner by the first optical waveguide and the first dual mode optical waveguide. Herein, the first optical waveguide can propagate higher-order modes as well as a fundamental mode therein. Furthermore, the first optical waveguide and the first dual mode optical waveguide are coupled with a mode discriminating directional coupler thereby causing an energy transfer between the fundamental mode of the first optical waveguide and the higher-order mode of the dual mode optical waveguide to occur with at least 50% efficiency and also the fundamental mode of the dual mode optical waveguide to propagate without change. In addition, the second dual mode optical waveguide for propagating optical signals which passed through the mode discriminating directional coupler therein is connected to the first dual mode optical waveguide. Furthermore, an arrangement of Bragg gratings is written within the second dual mode optical waveguide, in which at least one Bragg grating is connected in series which is tilted with an angle of more than 0.1 degree with respect to a plane perpendicular to the propagation direction of the optical signals. The Bragg grating serves to perform both mode conversion and reflection of the optical signals propagating within the second dual mode optical waveguide at drop wavelength xcexi and add wavelength xcexj.
In the present invention, an optical fiber or an integrated optics waveguide may be used as the first optical waveguide and the first and second dual mode optical waveguides. It is also preferred that the first and second dual mode optical waveguides are provided integrally without any connecting portions. When the first optical waveguide and the first and second dual mode optical waveguides are comprised of optical fibers, the fundamental mode of the first optical waveguide and the higher-order mode of the dual mode optical waveguides have same propagation constant, and more preferably the mode discriminating directional coupler is a fiber-optic directional coupler which utilizes the coupling of evanescent electric field. For the mode discriminating directional coupler, any of a polished-type directional coupler or a fused-type directional coupler may be used. In the mode discriminating directional coupler, a pair of a dual mode optical fiber and a single mode optical fiber is preferably selected which have propagation constants corresponding to each other suitably, so that an energy transfer occurs with at least 99% efficiency between LP01 mode of the single mode optical fiber and LP11 mode of the dual mode optical fiber, in which LP01 mode of the dual mode optical fiber never undergoes any energy transfer. In the mode discriminating directional coupler, the direction of the surface where the dual mode optical fiber and the single mode optical fiber contact together is preferably aligned with a plane perpendicular to the lobe direction of LP11 mode which propagates through the dual mode optical fiber, and the lobe direction of LP11 mode in the dual mode optical fiber is preferably determined along one of birefringence axes.
In the dual mode optical waveguide of the add-drop wavelength filter, LP01 mode is preferably used as the fundamental mode, and LP11 mode is perferably used as the higher-order mode, respectively. Furthermore, when using a dual mode optical fiber as the dual mode optical waveguide, the dual mode optical fiber preferably has linear birefringence so that the intensity distribution direction of the mode does not change in respect to the optical fiber while propagating in the optical fiber, Although a dual mode optical fiber having small birefringence may be used, in this case a polarization controller which can adjust the amount of the birefringence is preferably used together. In addition, the dual mode optical waveguide is preferably selected as an elliptic core optical fiber so that the energy distribution of the higher-order mode in the dual mode optical waveguide does not change as the optical signals propagate.
In the mean time, when the tilted Bragg grating is formed, the tilted direction of the Bragg grating is preferably selected to be the lobe direction of LP11 mode determined in the mode discriminating directional coupler; and is more preferably selected to be one of either an angle in which the fundamental mode to the fundamental mode reflection does not happen in the dual mode optical waveguide or an angle in which the higher-order mode to the higher-order mode reflection does not happen in the dual mode optical waveguide. When using an optical fiber as the optical waveguide, input is in the direction of the dual mode optical fiber of the mode discriminating directional coupler and output of the selected wavelength is in the direction of the single mode optical filter in case that the reflection between LP01 modes disappears, and vice versa in case that the reflection between LP11 modes disappears. The half power width of the reflection wavelength of the fiber-optic Bragg gratings is preferably less than 1 nm, and the reflectivity thereof is preferably more than 99%.
Furthermore, the add-drop wavelength filter may further comprise a mode converter which performs mode conversion between the fundamental mode and higher-order mode of said dual mode optical waveguides, and additionally a single mode optical waveguide may be connected in series to at least one of the first and second dual mode optical waveguides. In addition, an optical amplifier for amplifying for amplifying input/output optical signals may be preferably connected in series to at least one of the single mode optical waveguide and the first dual mode optical waveguide.
The method carrying out a wavelength add/drop by using the add-drop wavelength filter is as follows: First, when input optical signals are introduced into the the tilted Bragg gratings via the mode discriminating directional coupler, the tilted Bragg gratings reflect optical signals having a specific wavelength range while converting the mode of the optical signals. The reflected optical signals propagate through a port of the mode discriminating directional coupler which is different from the port where optical signals were input. This is same when performing add/drop of specific channels in respect to the optical signals, in which two of the mode discriminating directional couplers may be respectively connected to both sides of the Bragg grating thereby serving to add and drop channel signals.
When input optical signals are introduced into the mode discriminating directional coupler via the first optical waveguide, the entire signals propagate as converted into LP11 mode of the dual mode optical waveguide, and the signals are reflected at a specific wavelength which satisfies phase matching conditions while passing through the tilted Bragg gratings. These reflected signals are converted into LP01 mode and are output in the direction of the dual mode optical waveguide without undergoing any energy transfer in the mode discriminating directional coupler. On the contrary, when the input optical signals are introduced into the mode discriminating directional coupler as LP01 mode via the dual mode optical waveguide, the input optical signals propagate into the Bragg grating without undergoing any energy transfer and are reflected as converted into LP11 mode at the wavelength which satisfies phase matching conditions. These reflected optical signals are output to the first optical waveguide in the mode discriminating directional coupler. While passing through a second mode discriminating directional coupler, remaining optical signals which do not satisfy phase matching conditions are output through the first optical waveguide in case the input optical signals are introduced in the first optical waveguide and through the dual mode optical waveguide in case the input optical signals are introduced in the dual mode optical waveguide, respectively. In the dual mode optical waveguide within which a Bragg grating is written, a mode converter may be added at the opposite side with the reference of the Bragg grating from the position where the mode discriminating directional coupler is connected. Herein, the mode converter is a device which converts the fundamental mode signals into the higher-order mode signals which propagate through the dual mode optical waveguide. In the above mentioned case, when the input port is in the first optical waveguide direction, the remaining optical signals which do not satisfy phase matching conditions pass through the mode converter and propagate as converted into the fundamental mode of the dual mode optical waveguide. The configuration like this illustrates an apparatus for only separating wavelength. If the input optical signals are introduced into the mode converter as a fundamental mode, the optical signals which were converted into the higher-order mode pass through the Bragg grating and are output through the first optical waveguide by the mode discriminating directional coupler. At this time, if optical signals which satisfy phase matching conditions are introduced in the direction of the dual mode optical waveguide of the mode discriminating directional coupler, they are eventually output through the first optical waveguide. This corresponds to a wavelength adding apparatus. In summarizing, an add/drop wavelength filter may be constructed by combining a mode discriminating directional coupler, Bragg gratings and a mode converter.